The drilling of boreholes from subterranean locations through subterranean strata and structures is carried out for a wide variety of reasons and in many cases it is desirable to monitor the wall of the borehole or to detect possible faults or characteristics of the structure constituting the wall of the borehole.
Boreholes can also be drilled in oil and gas exploration, can be exploratory holes to determine the nature of subterranean structures and even can be holes of the type mentioned which are necessary in association with the mining of bituminous coal, for example.
Such holes can have lengths ranging from several meters to several hundred meters and even several kilometers in some cases.
An obvious way of monitoring a characteristic of the hole is to use a coring drill to extract a core of the material which can be evaluated to determine the particular structure at various depths, e.g. to determine the presence or absence of certain mineral matter.
Coring drilling cannot always be used, however, and direct monitoring of the nature of the structure via the drillings brought up by a drill is less satisfactory from the point of view of determining the subterranean structures through which the drilling is effected.
It is also desirable to evaluate the subterranean structures or production holes, i.e. bore holes drilled to extract natural gas. In bituminous coal mining and exploration, gas extraction and detection holes may also be drilled in the subterranean structure and such holes also may be drilled simply to explore the mining path or the vein or coal structure.
In general, therefore, considerable amounts of valuable information can be obtained by monitoring the wall structure of a borehole.
In large-bore technology, for example in the formation of large-diameter boreholes which are utilized for petroleum and natural gas production, it is already known to lower from the drilling site on the surface of the ground above the borehole a television camera into the latter together with an illuminating system so that the wall of the borehole is illuminated and a view can be taken thereof through a television system.
Because the camera must be relatively small and compact, and nevertheless provide high resolution, the cost of such systems, especially the camera, is relatively high.
Video cameras, moreover, generally require high-power lighting which likewise increases the cost and requires that the investigation be carried out independently from any other operations. In other words, such boring operations cannot be carried out with simultaneous drilling and other wall-monitoring investigations.
Finally, it should be noted that these techniques are not applicable at all with relatively small caliber bores of the type used in subterranean coal mining operations, e.g. when the drilling is effected from a subterranean location rather than from above the ground, and in other small-caliber subterranean drilling operations.